Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT) spread OFDM in the uplink. The basic LTE physical resource thus can be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element (RE) corresponds to one subcarrier during one OFDM symbol interval on a particular antenna port. An antenna port is defined such that a channel over which a symbol on the antenna port is conveyed can be inferred from a channel over which another symbol on the same antenna port is conveyed. There is one resource grid per antenna port. Notably, as discussed in Erik Dahlman et al., 4G LTE/LTE-Advanced for Mobile Broadband, §10.1.1.7 (2011), an antenna port does not necessarily correspond to a specific physical antenna but is instead a more general concept introduced, for example, to allow for beam-forming using multiple physical antennas. At least for the downlink, an antenna port corresponds to the transmission of a reference signal. Any data transmitted from the antenna port can then rely on that reference signal for channel estimation for coherent demodulation. Thus, if the same reference signal is transmitted from multiple physical antennas, these physical antennas correspond to a single antenna port. Similarly, if two different reference signals are transmitted from the same set of physical antennas, this corresponds to two separate antenna ports.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms), where each radio frame consists of ten equally-sized subframes of 1 ms as illustrated in FIG. 2. A subframe is divided into two slots, each of 0.5 ms time duration. Resource allocation in LTE is described in terms of Resource Blocks (RBs), or Physical RBs (PRBs), where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kilohertz (kHz) subcarriers in the frequency domain. Two consecutive resource blocks in the time domain represent a resource block pair and correspond to the time interval upon which scheduling operates.
Transmissions in LTE are dynamically scheduled in each subframe where a base station transmits downlink assignments/uplink grants to certain User Elements, or User Equipments, (UEs) via a Physical Downlink Control Channel (PDCCH) and, starting in LTE Release 11 (Rel-11), an enhanced PDCCH (ePDCCH). PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and span (more or less) the whole system bandwidth. A UE that has decoded a downlink assignment, carried by a PDCCH, knows which resource elements in the subframe that contain data aimed for the UE. Similarly, upon receiving an uplink grant, the UE knows which time/frequency resources it should transmit upon. In the LTE downlink, data is carried by a Physical Downlink Shared Channel (PDSCH). In the uplink, the corresponding link is referred to as a Physical Uplink Shared Channel (PUSCH).
Definition of the ePDCCH is ongoing in 3GPP. It is likely that such control signaling will have similar functionalities as PDCCH. However, a fundamental difference for ePDCCH is that ePDCCH will require UE specific reference signals (i.e., Demodulation Reference Signals (DMRS)) instead of cell specific reference signals (i.e., Common Reference Signals (CRS) for its demodulation. One advantage is that UE specific spatial processing may be exploited for ePDCCH.
Demodulation of data sent via the PDSCH requires estimation of large-scale channel properties of the radio channel. This channel estimation is performed using transmitted reference symbols, where reference symbols are symbols of a Reference Signal (RS) and are known to the receiver. In LTE, CRS reference symbols are transmitted in all downlink subframes. In addition to assisting downlink channel estimation, the CRS reference symbols are also used for mobility measurements performed by the UEs. LTE also supports UE specific RS reference symbols aimed only for assisting channel estimation for demodulation purposes. FIG. 3 illustrates one example of mapping of physical control/data channels and signals onto resource elements within a RB pair forming a downlink subframe. In this example, PDCCHs occupy the first out of three possible OFDM symbols. So, in this particular case, the mapping of data could start at the second OFDM symbol. Since the CRS is common to all UEs in the cell, the transmission of the CRS cannot be easily adapted to suit the needs of a particular UE. This is in contrast to UE specific RSs where each UE has a UE specific RS of its own placed in the data region of FIG. 3 as part of the PDSCH.
The length of the control region, which can vary on a subframe basis, is conveyed in the Physical Control Format Indicator Channel (PCFICH). The PCFICH is transmitted within the control region at locations known by the UEs. After a UE has decoded the PCFICH, the UE knows the size of the control region and in which OFDM symbol the data transmission begins. A Physical Hybrid-Automatic Repeat Request (HARQ) indicator, which carries ACK/NACK responses to a UE to inform the UE of whether a corresponding uplink data transmission in a previous subframe was successfully decoded by the base station, is also transmitted in the control region.
In LTE Release 10 (Rel-10), all control messages to UEs are demodulated using the CRSs. Therefore, the control messages have cell wide coverage to reach all UEs in the cell. An exception is the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS), which are stand-alone and do not need reception of a CRS before demodulation. The first one to four OFDM symbols in a subframe, depending on the configuration, are reserved for such control information. Control messages can be categorized into control messages that need to be sent only to one UE in the cell (i.e., UE-specific control messages) and control messages that need to be sent to all UEs in the cell or some subset of the UEs in the cell numbering more than one (i.e., common control messages).
As illustrated in FIG. 4, control messages of the PDCCH type are demodulated using CRSs and transmitted in multiples of units called Control Channel Elements (CCEs), where each CCE contains 36 REs. A PDCCH may have an Aggregation Level (AL) of 1, 2, 4, or 8 CCEs to allow for link adaptation of the control message. Furthermore, each CCE is mapped to 9 Resource Element Groups (REGs) consisting of 4 REs each. These REGs are distributed over the whole system bandwidth to provide frequency diversity for a CCE. Hence, a PDCCH, which consists of up to 8 CCEs, spans the entire system bandwidth in the first one to four OFDM symbols, depending on the configuration.
In LTE Rel-11, it has been agreed to introduce UE-specific transmission of control information in the form of enhanced control channels. More specifically, it has been agreed to allow transmission of generic control messages to a UE using transmissions based on UE-specific RSs placed in the data region. This is commonly known as an ePDCCH, an enhanced Physical HARQ Indicator Channel (ePHICH), etc. FIG. 5 illustrates a downlink subframe showing 10 RB pairs and configuration of three ePDCCH regions of size 1 RB pair each. The remaining RB pairs can be used for PDSCH transmissions. For ePDCCH in LTE Rel-11, it has been agreed to use antenna port pε{107, 108, 109, 110} for demodulation as illustrated in FIG. 6 for normal subframes and normal cyclic prefix. More specifically, FIG. 6 illustrates an example of RE locations for UE-specific reference symbols (i.e., DMRS reference symbols) used for ePDCCH in LTE for one PRB pair. Note that, starting in LTE Rel. 11, more than one UE can, in some cases, unknowingly of each other use the same DMRS reference symbols to demodulate their respective ePDCCH messages. As such, “UE specific” should be interpreted as seen from the UEs perspective. RS ports R7 and R9 represent the DMRS reference symbols corresponding to antenna port 107 and 109, respectively. In addition, antenna ports 108 and 110 can be obtained by applying an orthogonal cover (1, −1) over adjacent pairs of RS ports R7 and R9, respectively. The ePDCCH enables precoding gains to be achieved for the control channels. Another benefit of the ePDCCH is that different PRB pairs (or enhanced control regions) can be allocated to different cells or different transmission points within a cell and, as such, inter-cell or inter-point interference coordination between control channels can be achieved. This is especially useful for heterogeneous network scenarios, as discussed below.
The concept of a point is heavily used in conjunction with techniques for Coordinated Multi-Point (CoMP). In this context, a point corresponds to a set of antennas covering essentially the same geographical area in a similar manner. Thus, a point might correspond to one of multiple sectors at a site (i.e., one of two or more sectors of a cell served by an enhanced Node B (eNB)), but it may also correspond to a site having one or more antennas all intending to cover a similar geographical area. Often, different points represent different sites. Antennas correspond to different points when they are sufficiently geographically separated and/or have antenna diagrams pointing in sufficiently different directions. Techniques for CoMP entail introducing dependencies in the scheduling or transmission/reception among different points, in contrast to conventional cellular systems where a point from a scheduling point of view is operated more or less independently from the other points. Downlink CoMP operations may include, for example, serving a certain UE from multiple points, either at different time instances or for a given subframe, on overlapping or non-overlapping parts of the spectrum. Dynamic switching between transmission points serving a certain UE is often termed as Dynamic Point Selection (DPS). Simultaneously serving a UE from multiple points on overlapping resources is often termed as Joint Transmission (JT). Point selection may be based on, for example, instantaneous conditions of the channels, interference, or traffic. CoMP operations are intended to be performed for data channels (e.g., PDSCH) and/or control channels (e.g., ePDCCH).
The same ePDCCH region can be used by different transmission points within a cell or belong to different cells that are not highly interfering with respect to one another. A typical case is the shared cell scenario illustrated in FIG. 7. As illustrated, a heterogeneous network includes a macro node, or macro base station, and multiple lower power pico nodes, or pico base stations, within a coverage area of the macro node. The same ePDCCH region can be used by the macro node and the pico nodes. Note that, throughout this application, nodes or points in a network are often referred to as being of a certain type, e.g., “macro” or “pica.” Unless explicitly stated otherwise, this should not be interpreted as an absolute quantification of the role of the node/point in the network but rather as a convenient way of discussing the roles of different nodes/points relative each other. Thus, a discussion about macro and pico nodes/points could for example just as well be applicable to the interaction between micro and femto nodes/points.
For pico nodes that are geographically separated, such as pico nodes B and C, the same ePDCCH region can be re-used. In this manner the total control channel capacity in the shared cell will increase since a given PRB resource is re-used, potentially multiple times, in different parts of the cell. This ensures that area splitting gains are obtained. An example is given in FIG. 8 where pico nodes B and C share the same ePDCCH regions. Conversely, due to proximity, pico nodes A and B and pico nodes A and C are at risk of interfering with each other and, therefore, pico node A is assigned an ePDCCH region that is non-overlapping with the shared ePDCCH regions of the pico nodes B and C. Interference coordination between pico nodes A and B, or equivalently transmission points A and B, within the shared macro cell is thereby achieved. Likewise, interference coordination between pico nodes A and C, or equivalently transmission points A and C, within the shared macro cell is thereby achieved. In some cases, a UE may need to receive part of the ePDCCH signaling from the macro cell and the other part of the ePDCCH signaling from the nearby pico cell. This area splitting and control channel frequency coordination is not possible with the PDCCH since the PDCCH spans the whole bandwidth. Also, the PDCCH does not provide possibility to use UE specific precoding since it relies on the use of CRS for demodulation.
FIG. 9 illustrates an ePDCCH that, similar to the CCE in the PDCCH, is divided into multiple groups and mapped to one of the enhanced control regions of a subframe. Note that in FIG. 9, the ePDCCH regions do not start at OFDM symbol zero in order to accommodate simultaneous transmission of a PDCCH in the subframe. However, there may be carrier types in future LTE releases that do not have a PDCCH, in which case the ePDCCH regions could start from OFDM symbol zero within the subframe.
Even if ePDCCH enables UE specific precoding and localized transmission as discussed above, it can, in some cases, be useful to be able to transmit ePDCCH in a broadcasted, wide area coverage fashion. This is useful if the base station (i.e., eNB) does not have reliable information to perform precoding towards a certain UE. In this situation, a wide area coverage transmission is more robust. Another case is when the particular control message is intended for more than one UE. In this case, UE specific precoding cannot be used. An example is the transmission of the common control information using PDCCH (i.e., in the Common Search Space (CSS)). In any of these cases, a distributed transmission over multiple ePDCCH regions within a subframe can be used. One example of such distribution is illustrated in FIG. 10 where the four parts belonging to the same ePDCCH are distributed over multiple enhanced control regions within a subframe. It has been agreed in the 3GPP ePDCCH development that both distributed and localized transmission of an ePDCCH should be supported. When distributed transmission of ePDCCH is used, it is also beneficial if antenna diversity can be achieved to maximize the diversity order of an ePDCCH message. On the other hand, sometimes only wideband channel quality and wideband precoding information is available at the base station, in which case it could be useful to perform a distributed transmission but with UE specific, wideband precoding.
As discussed above, enhanced control signaling, such as ePDCCH in LTE, offers many advantages. However, advanced network architectures (e.g., heterogeneous network architectures) and downlink CoMP lead to issues that must be solved. In particular, as discussed below, the inventors have found that there is a need for systems and methods for improved channel estimation techniques.